A number of cell surface receptor proteins, referred to as "cell adhesion receptors" (CARs) have been identified that bind to extracellular matrix ligands or other cell adhesion protein ligands thereby mediating cell-cell and cell-matrix adhesion processes. The CARs are encoded by genes belonging to a gene superfamily and are composed of heterodimeric transmembrane glycoproteins containing .alpha.- and .beta.-subunits. CAR subfamilies contain a common .beta.-subunit combined with different .alpha.-subunits to form adhesion protein receptors with different specificities. The genes for at least eight distinct .beta.-subunits have been cloned and sequenced to date.
One of the larger classes of CARs includes the integrins, which comprise inter alia perhaps the most significant CAR, that is glycoprotein IIb/IIIa ("GPIIb/IIIa" or "IIb/IIIa"), GP IIb/IIIa is also referred to as the fibrinogen receptor, and is the principal membrane protein mediating platelet aggregation. GP IIb/IIIa in activated platelets is known to a group of soluble proteins defined by their common amino acid motif Arg-Gly-Asp (RGD). These proteins include fibrinogen, von Willebrand factor, fibronectin, and vitronectin. The RGD recognition sequence is important to the binding of these proteins to the IIb/IIIa receptor, as well as other integrins.
As noted, a number of cell surface cell adhesion receptors have been identified and their functions are being elucidated. For example, certain members of the .beta..sub.1 subfamily, i.e., .alpha..sub.4 .beta..sub.1 and .alpha..sub.5 .beta..sub.1, have been implicated in various inflammatory processes, including rheumatoid arthritis. In addition, studies with monoclonal anti-.alpha..sub.4 antibodies provide evidence that .alpha..sub.4 .beta..sub.1 may additionally have a role in allergy, asthma, and autoimmune disorders. Anti-.alpha..sub.4 antibodies block the migration of leukocytes to the site of inflammation.
The .alpha..sub.v .beta..sub.3 integrin, also referred to as the vitronectin receptor, is a heterodimer and is a member of the .beta..sub.3 integrin subfamily. The .alpha..sub.v .beta..sub.3 integrin is found on platelets, endothelial cells, melanoma cells, smooth muscle cells, and osteoclasts. Like the CAR IIb/IIIa, the .alpha..sub.v .beta..sub.3 integrin binds a variety of RGD-containing adhesive proteins such as vitronectin, fibronectin, von Willebrand factor, fibrinogen, osteopontin, bone sialo protein II and thrombospondin, again mediated by the RGD sequence. Thus, .alpha..sub.v .beta..sub.3 acts as the endothelial cell, fibroblast, and smooth muscle cell receptor for adhesive proteins including von Willebrand factor, fibrinogen (fibrin), vitronectin, thrombospondin, and osteopontin.
The .alpha..sub.v .beta..sub.3 integrin allows endothelial cells to interact with a wide variety of extracellular matrix components. These adhesive interactions are considered to be important for angiogenesis since vascular cells must ultimately be capable of invading virtually all tissues. .alpha..sub.v .beta..sub.3 is also involved in bone resorption since a key event in bone resorption is the adhesion of osteoclasts to the matrix of bone. As a consequence of injury to the endothelium, the basement membrane zones of blood vessels express several adhesive proteins, including von Willebrand factor, fibronectin, and fibrin. Additionally, several members of the integrin family of adhesion protein receptors are expressed on the surface of endothelial, smooth muscle and on other circulating cells. Among these CARs is the .alpha..sub.v .beta..sub.3 integrin. These CARs initiate a calcium-dependent signaling pathway that can lead to endothelial cell, smooth muscle cell migration and, therefore, may play a fundamental role in vascular cell biology.
Several cell adhesion inhibitory molecules, that act as CAR antagonists, are currently being investigated as drug candidates. Inhibitors of .alpha..sub.v .beta..sub.3 have been shown to inhibit angiogenesis and are recognized as being useful as therapeutic agents for the treatment of human diseases such as cancer, restenosis, thromboembolic disorders, rheumatoid arthritis and ocular vasculopathies. The binding of fibrinogen and von Willebrand factor to the RGD-binding domain of GP IIb/IIIa causes platelets to aggregate. RGD-peptidomimetic IIb/IIIa antagonist compounds are known to block fibrinogen binding and prevent platelet aggregation and the formation of platelet thrombi. Therefore, IIb/IIIa antagonists represent an important new approach for antiplatelet therapy for the treatment of thromboembolic disorders. See, for example, the discussion of .alpha..sub.v .beta..sub.3 antagonists in Lefkovitz J et al., "Platelet glycoprotein IIb/IIIa receptors in cardiovascular medicine," New Engl J Med 332:1553-1559 (1995), indicating the potential utility of such compounds in the treatment of various disease states, e.g., restenosis, unstable angina, stroke, prevention of secondary myocardial infarction, etc.
The usefulness of pharmacological intervention at the level of the cell adhesion receptor has already been demonstrated with the commercially available murine monoclonal antibody Abciximab sold under the trade name ReoPro.TM.. This agent is directed against the human GPIIb/IIIa receptor, and is currently marketed as an intravenous infusion for prevention of restenosis following angioplasty.
Smaller molecules that bind to platelets at the GPIIb/IIIa receptor are currently being developed as intravenous infusion, oral, and passive transdermal preparations. To be effective and safe, such agents would have to be administered, to target plasma levels, continuously with little variation in blood level concentrations. This is required since the therapeutic window for the drug is likely to be narrow. Consequently, intravenous infusions of these drugs would be ideal from a pharmacokinetic viewpoint. Unfortunately, prolonged intravenous infusion is both costly and impractical, particularly if these agents have to be given chronically in the home setting.
Oral dosing of such agents has the advantage of being well-tolerated. However, to be useful an oral agent would have to be found with a relatively long elimination plasma half-life, as a short elimination plasma half-life would necessitate frequent daily dosing. In addition, oral bioavailability would need to be relatively high and not be affected by food, alcohol, etc., to avoid variations in dosing.
Ester prodrugs are often developed to improve the oral bioavailability of poorly absorbed drugs. Such prodrugs are rapidly broken down by hydrolysis or through esterase metabolism to the parent carboxylic acid. For certain drugs, such as the narcotic analgesic remifentanil, the ester function allows the drug to be rapidly metabolized to less active metabolites and, therefore, its pharmacological action can be rapid when given by intravenous infusion. For prodrugs, ester degradation is desirable to allow for transformation to the active agent. However, certain drugs that have narrow therapeutic windows and/or that require continuous delivery are unsuitable for oral delivery. A viable alternative to intravenous infusion or oral delivery of medicaments is transdermal delivery, which has recently become acceptable and increasingly important means of administering drugs.
Presently there are two types of transdermal drug delivery systems, i.e., "passive" and "active." In passive transdermal systems chemical potential gradients provide the dominant driving force to deliver the drug through the skin. For these drugs, a patch containing the drug is applied to the surface of the body and the drug moves into the body predominantly driven by difflusion controlled transport. Passive transdermal delivery has been shown to be an effective and convenient form for delivering a number of molecules. Some examples of passive transdermal systems include: delivery of nicotine, nitroglycerine, scopolamine, clonidine, fentanyl, testosterone, estradiol, etc. However, this method of delivery may not be amenable for certain ester drugs since it has been shown that the skin contains esterases that are sufficient to metabolize topically applied esters. See, Zhou XH and Li Wan Po A, "Comparison of enzymatic activities of tissues lining portals of drug absorption, using the rat as the model," Int J Pharmacol 62:259-267 (1990). Additionally, passive transdermal delivery is only really effective for delivery of potent small molecules that are relatively lipophilic.
Passive transdermal administration is also unlikely to provide enough drug input, for these agents are typically charged and hydrophilic in nature and therefore would not expect to permeate readily across the lipophilic outermost layer of the skin. Chemical enhancers have also been employed to improve passive transdermal delivery of GPIIb/IIIa receptor inhibitors, see WO 95/13825 to Feigen L. P. and Griffen M. J., entitled: "Transdermal compositions of N-N-5-4(aminoethyl)phenyl!-1-oxopentyl!-L-phenylalanine or its ester and their pharmaceutically acceptable salts," and incorporated herein by reference. However, the variability associated with this delivery technique is likely to be too high for drugs with high or narrow therapeutic windows, such as cell adhesion molecules.
The second type of transdermal drug delivery is active transdermal delivery. In active transdermal systems, additional, extrinsically applied driving forces, either electrical (iontophoresis) or ultrasonic (phonophoresis), are used to control delivery of the drugs through the skin.
Iontophoresis, according to Stedman's Medical Dictionary, is defined as "the introduction into the tissues, by means of an electric current, of the ions of a chosen medicament." Iontophoretic devices have been known since the 1900's to be an effective means of delivery of hydrophilic and charged drugs across the skin and into the systemic circulation. In iontophoretic transdermal systems applied electric potential provide the dominant driving force to deliver the ionized drug through the skin. For these drugs, an iontophoretic patch containing the drug is applied to the surface of the body, controlled current is driven through the patch via electrodes in contact with the patch and the drug moves into the body predominantly driven by migration controlled transport. Some examples of iontophoretic transdermal systems include: delivery of pilocarpine in diagnosing cystic fibrosis, delivery of topical anesthetic to name a few.
The iontophoretic patch primarily consists, at a minimum, of two compartments, an anode and a cathode, each of which is individually in contact with the body. The electrode compartments house the electrodes in contact with the ionic media and are disposed to be in intimate ionic contact with some portion of the body through the skin, to complete the internal electrical circuit. The electrodes are connected externally to a power supply to complete the external electrical circuit. During operation the entire system, power source, electrode, electrolyte, the skin and the body, forms one integrated electrochemical cell.
The electrode connected to the positive pole of the power supply is called the anode and the electrode connected to the negative pole of the power supply is called the cathode. When the current is turned on at the power supply, current flows from the anode to the cathode in the system controlled externally (to the patch) by electron transport and internally (inside the patch between the electrodes) by ion transport. This is possible because the electrodes act as transducers converting electron transport to ion transport via an electron transfer reaction (electrochemical reaction) at the electrode.
In general positive ions (cations) will tend to carry portion of the current and move towards the cathode and the negative (anions) ions will tend to carry portion of the current and move towards the anode. Hence by loading cationic drugs in the anode compartment and/or anionic drugs in the cathode compartment, iontophoresis can be used to deliver the ionized drug across the skin separator into the body.
In general, the flux of a drug across the skin from an iontophoretic device is directly proportional to the applied current. Thus, a way to obtain varied flux or drug delivery profiles would be to vary the current. By way of example, if one wanted to administer a bolus-like (or peaked) flux, one would need to increase the current at first and then decrease the current after the bolus has been achieved.
The iontophoresis process has been found to be useful in the transdermal administration of therapeutic drugs including lidocaine hydrochloride, hydrocortisone, fluoride, penicillin, dexamethasone sodium phosphate, insulin and other drugs. A common use of iontophoresis is in the diagnosis of cystic fibrosis by delivering pilocarpine salts iontophoretically, where the pilocarpine stimulates sweat production and the sweat is collected and analyzed for its chloride content to detect the presence of the disease.
As mentioned above, for a drug to be iontophoresed across the skin effectively it must be ionizable. In addition, it has been found that the drug must be able to maintain its charge during its passage across the epidermis. See, for example, U.S. Pat. No. 5,494,679 to Sage et al., entitled "Molecules for iontophoretic delivery," the entire disclosure of which is incorporated herein by reference. For positively charged ester compounds, metabolism of the positively charged ester compound in the skin will result in the exposure of one or more charged carboxylic acid groups on the compound and, therefore, a neutralization or reversal of the positive charge on the compound as it is iontophoresed across the skin. One of ordinary skill in the art would expect that such metabolism would result in poor mobility and irregular delivery of the compound via an iontophoretic route.